Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power generation, networks, computers, and consumer products. Semiconductor devices are also found in electronic products including military, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The atomic structure of semiconductor material allows its electrical conductivity to be manipulated by the application of an electric field or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including transistors, control the flow of electrical current. By varying levels of doping and application of an electric field, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, diodes, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed calculations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing, and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual die from the finished wafer and packaging the die to provide structural support and environmental isolation.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller die size may be achieved by improvements in the front-end process resulting in die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
One common technique of interconnecting a semiconductor die with a printed circuit board or other device involves the use of solder bumps. FIG. 1 shows a conventional solder bump structure. A conductive layer 3 is formed over substrate 2. Conductive layer 3 is a metal pad. An insulating or dielectric layer 4 is formed over substrate 2 and conductive layer 3. A portion of insulating layer 4 is removed by an etching process to expose conductive layer 3. An insulating or passivation layer 5 is formed over conductive layer 3 and insulating layer 4. A portion of insulating layer 5 is removed by an etching process to expose conductive layer 3. A conductive layer 6 is formed over conductive layer 3 and insulating layer 5. Conductive layer 6 is a redistribution layer to route electrical signals from conductive layer 3 to a later-formed solder bump. An insulating or passivation layer 7 is formed over conductive layer 6 and insulating layer 5. A portion of insulating layer 7 is removed by an etching process to expose conductive layer 6. A conductive layer 8 is formed over conductive layer 6 and insulating layer 7. Conductive layer 8 is an under bump metallization (UBM) layer for the solder bump. UBM 8 is substantially flat. FIG. 1b shows a cross-sectional view across UBM 8. Solder bump 9 is formed on UBM 8.
The joint interface area between UBM 8 and solder bump 9 is important to the solder joint reliability. A large UBM pad size generally results in more metal contact area and better solder joint reliability. However, if the UBM pad is too large in comparison to the solder bump height, the solder joint can develop voids which can cause product defects. In addition, a large UBM pad consumes area which is counter to the general goal of smaller feature size.